Accurate detection of abnormalities in a patient's body is an essential prerequisite for diagnosing and adequately treating disease. Visualization methods, e.g., magnetic resonance imaging (MRI), are becoming more important for such accurate detection. MRI is non-invasive and requires no exposure of humans to potentially harmful radiation. In MRI, tissues of different origin, such as normal and deviated, e.g., cancerous, tissues, may be differentiated on the basis of differences in relaxation times T1, the spin-lattice or longitudinal relaxation time, or T2, the spin-spin or transverse relaxation time. Because of these differences, differential signal intensity is produced which gives various degrees of contrast in MR images. The greater the difference in T1 or T2, the more pronounced the contrast. However, in many cases diseased or deviated tissue is isointensive, i.e., the diseased or deviated tissue has the same signal intensity as normal tissue, and is therefore not distinguishable from normal tissue without the use of special contrast agents.
Where MR imaging techniques employed to elucidate blood perfusion defects are based on the differentiation of flowing blood from stationary surrounding tissues, e.g., MR angiography (MRA). Three dimensional angiographic techniques, e.g., "Time of Flight" (TOF) and "Phase Contrast" (PC) techniques, provide detailed images of intracranial vessels. However, traditional MRA, i.e., Time of Flight MRA, is dependent on flow velocity and flow shape and thus high-quality angiography of peripheral vessels with high flow resistance is generally impossible due to an effect known as vessel saturation. To overcome this problem, contrast agents have been used to selectively lower the relaxation times of blood.
Gadolinium (III) diethylenetriamine pentaacetic acid (Gd-DTPA) dimeglumine is a widely used contrast agent which is relatively small (MW 538) and extravasates on the first pass through the capillaries. However, the use of Gd-DTPA for MR angiography in all organs except the brain is limited, since the blood half-life of Gd-DTPA is less than 20 minutes, and the biological life in man of GD-DTPA is about 90 minutes. The extravasation results in a rapid decrease in vessel/muscle signal ratio, which makes the accurate detection of abnormalities and disease difficult. Moreover, Gd-DTPA dimeglumine, which is used in clinical practice, is immunogenic, which does not favor its repetitious administration to the same patient.
Similar problems occur with the use of ferrioxamine-B as a contrast agent. In addition, ferrioxamine-B causes a precipitous drop in blood pressure after its intravenous administration.
MRI contrast agents created using natural and synthetic macromolecules offer the advantage of high molecular relaxivity due to the multiple chelating groups coupled to a single polymer backbone. These groups can chelate paramagnetic cations, e.g., in Gd-DTPA-poly-l-lysine, or produce high relaxivity due to the presence of iron oxide, e.g., in iron-containing colloids. However, iron oxide-based colloids have their own ligand-independent specific site of accumulation in the body, e.g., the liver, spleen, and lymphoid tissues.
Chelating groups may be attached to a variety of natural polymers, e.g., proteins and polysaccharides, and synthetic polymers. Chemical attachment, e.g., by conjugation, of DTPA to bovine serum albumin will result in a macromolecular contrast agent, which is suitable for some applications, e.g., NMR-angiography, but because of the efficient recognition of modified albumin by macrophages, and albumin-receptors on endothelial cells this contrast agent has a short blood half-life. It is also immunogenic and toxic to reticuloendothelial system organs. Therefore, use for MR imaging is limited.
One way to diminish the antigenicity of albumin is to mask it with natural and synthetic polymers, e.g., spacer arms, by covalent attachment, but this leaves few reactive groups in the protein globule which are needed for binding the chelates and paramagnetic cations. Therefore, the use of such complexes in MR imaging is limited.
Synthetic polymers of 1-amino acids, such as poly-l-lysine (PL), are an alternative to modified natural proteins as backbones for contrast agents. PL modified with DTPA can be used as a radionuclide carrier for antibody-mediated targeting in nuclear medicine. Poly-l-lysine-DTPA, i.e., poly-l-lysine with DTPA groups bonded to epsilon-amino groups of lysine residues has been suggested as a Gd complexone, i.e., a compound which forms a complex with Gd, for use in MR angiography. It is also known that the toxicity of DTPA-poly-l-lysine is lower than that of DTPA-albumin. However, DTPA-moieties on DTPA-polylysine are recognized by liver Kupffer cells and some kidneys cells, presumably glomerulonephral phagocytes, which cause elevated and relatively rapid removal of the contrast agent from the blood. For example, 90% of the intravenously injected agent, e.g., poly-l-lysine-DTPA(Gd) (MW 48.7 kD), is removed from circulation in 1 hour (t.sub.1/2 =0.134 h) and accumulated in the kidneys, liver, and bone. Moreover, synthesis of DTPA-poly-l -lysine can be carried out with a cross-linking reagent, e.g., cyclic anhydride of DTPA. As a result, it is difficult to avoid the formation of cross-linked products of relatively high molecular weight and the preparation obtained is heterogeneous.
Nitrogen-containing polymers, e.g., polethyleneimine, have been modified with monofunctional derivatives of acetic acid to form a molecule where the backbone nitrogens and acetic acid residues are involved in complex formation with trivalent cations. However, because of extensive undesirable accumulation in the liver, paramagnetic complexes of polyethyleneiminoacetic acid are not widely used in MRI.
Polymeric contrast agents, e.g., starburst dendrimers, constitute a separate family of macromolecules with limited potential value as contrast agents. This family of agents has not been shown to be biocompatible and thus its value for in vivo imaging is limited.
Various polysaccharide-based chelating agents have been previously described; however, their activation complement which has been shown to be a feature of polysaccharides, preclude their extensive use in MR imaging.